1. Field of the Invention
The invention relates to the removal of pollutants from a gaseous stream and, more particularly, to a method and apparatus for filtering particulate matter from an exhaust gas.
2. Discussion of Prior Art
Industrial processes often involve the combustion of a fuel within a boiler to generate heat. Examples of the fuel include coal, wood, oil, natural gas or other suitably combustible material that is traditionally combusted in the presence of air or another oxygen source within the boiler. The heat generated from combustion of the fuel can be used to convert water into steam for driving a turbine, to heat an enclosure, or to perform another desired operation.
Boilers and other combustion sources of heat emit particulate and other pollutants entrained in their exhaust gas. Previous attempts to remove the pollutants from the exhaust emitted by a boiler have involved subjecting the exhaust gas to electrostatic precipitation followed by filtering the remaining pollutants from the exhaust gas using a fabric filter. Because electrostatic precipitation is effective in removing coarse pollutants the fabric filter of conventional systems has typically been exposed to fine particulate pollutants that are too small to be removed using electrostatic precipitation. To minimize the quantity of the fine particulate pollutants escaping the fabric filter the pore size of such filters can be designed as small as possible. However, the particle size of the fine particulate pollutants removed by the fabric filter is sufficiently small to form a dense dust cake on the fabric filter that eventually creates an unacceptably large pressure drop. To remove this dust cake high-pressure pulses of air or other cleaning gas are injected into the fabric filters. However, the high-pressure pulses can fatigue the fabric over time, thereby reducing the life of the fabric filters.
An example of a system to increase the particle size of the pollutants reaching the fabric filter to minimize the effect of cake formation can be found in U.S. Pat. No. 7,300,496 to Taylor, which is incorporated in its entirety herein by reference. Such a system involves the use of an agglomerator to combine fine particulate pollutants into larger masses that are then removed by the fabric filter. However, such agglomerator systems have traditionally been designed to treat the particulate matter that is embedded in the gas stream with the agglomerator positioned down stream of a primary collection device such as an electrostatic precipitator but upstream of a secondary filter. In the past, system designs did not take into account the injection of sorbents, for multi pollutant (examples being particulate, mercury, NOx, HCl, etc) reduction such as activated carbon, lime, etc downstream of a primary collection device such as an electrostatic precitator, multiclone, baghouse, etc but upstream of an agglomerator that in turn is positioned upstream of a secondary filter. In such a system that does not include the agglomerator technology, injected sorbents have been found to increase the rate of bag blinding and increase the system pressure drop.
Additionally, sorbent injection systems designed without the use of an agglomerator have been found not to achieve the same level of mixing of the sorbents in the gas flow. This poor mixing reduces the effectiveness of the sorbent to capture the targeted pollutants in the gas stream, allowing a sorbent-enriched gas stream including exposed sorbent to enter the fabric filter. The excess sorbent not combined with the particulate pollutants collects on the fabric filters, again causing filter blinding and a large pressure drop to be experienced by the system. Further, the adhesion between the fabric filters and the pollutants caused by the injected sorbent for multipollutant reduction makes cleaning the fabric filters difficult.